This invention relates to a novel method for etching precisely-sized and shaped apertures into a strip of nickel-iron alloy; for example, invar steel. The etched product may be used to make shadow masks for color television picture tubes, as well as other precision-etched products.
A common type of color television picture tube comprises an evacuated glass envelope having a viewing window, a luminescent viewing screen supported by the inner surface of the viewing window, a formed shadow mask closely spaced from the viewing screen and an electron-gun mount assembly for generating one or mre electron beams for selectively exciting the screen to luminescence. The formed shadow mask, which is a thin metal membrane having precisely-sized and shaped apertures therethrough, is used as a photographic master for making the screen, and then is used, during the operation of the tube, to aid in color selection on the screen by shadowing the electron beams. In both of these functions, it is important that the apertures therein follow closely in sizes and shapes with the mask specifications.
A flat mask is ordinarily made in several steps including producing etch-resistant stencils on opposite surfaces of a strip of low-carbon steel and then etching through the stencilled strip with a ferric-chloride etchant. The flat mask is then removed from the strip and formed to a desired shape. The strip is ordinarily about 0.10 to 0.20 mm (4-8 mils) thick, and the apertures therein may be round or slit shaped and may range in diameter or width from about 0.25 mm (10 mils) to less than the thickness of the strip. In addition, the profiles of the apertures are tapered so as to reduce scattering of electrons during tube operation. Each aperture has a "knife edge" at its smallest diameter or width which defines the shape of the electron beamlet passing therethrough. That smallest diameter should be precisely shaped, and the tapered surface should be as smooth as possible to achieve this feature and also to reduce electron scattering.
For some picture tubes, it is desirable to fabricate the shadow mask from a strip of invar steel, which is an alloy containing about 36% nickel and about 64% iron. Invar steel masks have a relatively low coefficient of thermal expansion and greater structural strength than low-carbon steel. Thereby, an invar steel mask exhibits less dimensional change when it is heated during tube operation and also can be made from thinner strip material.
The parameters to be controlled during the etching phase for low-carbon-steel shadow masks are well known in the art. These include control of etchant temperature, Baume (specific gravity), redox potential, free-acid concentration, line speed, spray pressure and location of spray nozzles with respect to the metal strip in the etch chamber. However, when etching invar steel masks an additional parameter is introduced, the nickelous ion (Ni.sup.2+) concentration in the etchant. This results from the oxidation of elemental nickel in the invar alloy via the reduction of ferric ions in the etchant. With continued etching, the nickel ion concentration in the ferric-chloride etchant will increase until it approximates the 36 weight percent composition of the invar alloy. since nickelous ion will not oxidize elemental iron, or nickel in the invar alloy, or be further transformed to a higher oxidation state by chlorine gas, it contaminates and dilutes the ferric-chloride etchant. This has several deleterious effects upon the etching process. It slows the etch rate (and hence productivity) of the etch line and produces rough etch resulting in poor visual uniformity in the finished mask.
It is to be understood that by rough or smooth etch we refer to the surface roughness of the metal on the inside of the apertures in the shadow mask. A surface roughness of .ltoreq.10 microinches (smooth etch) results in a mask with good visual uniformity. Increases above this value in surface roughness (rough etch) contribute to a general decline in visual uniformity to transmitted light in the finished mask. This, in turn, degrades the ambient appearance of the phosphor screen produced with the mask, and also the white uniformity of the screen in an operating picture tube.
Some of the foregoing problems are addressed in U.S. Pat. No. 4,420,366 issued Dec. 13, 1983 to K. Oka et al., which discloses a method for etching nickel-iron shadow masks. The method disclosed in the Oka et al. patent requires the ferric-chloride etchant to have a controlled content of free hydrochloric acid, a temperature in the 40.degree. to 70.degree. C. range, a total amount of nickelous and ferrous irons not more than 15 weight percent and a specific gravity in about the 1.420 to 1.540 range (43.degree. to 51.degree. Baume) depending upon the temperature of the etchant. Our data indicates that the problems are only partly overcome by the method disclosed in the Oka et al. patent, and that further significant improvements are possible. The novel method permits more accurate control of the etching step using ordinary process-control equipment, while consistently producing masks within close specification. Furthermore, the novel method is economical in etching time and in the consumption of etchant.